Endogenous pulmonary surfactant reduces surface tension at the air-liquid interface of the alveolar lining, preventing the lungs from collapsing at end expiration. Surfactant deficiency is a common disorder in premature infants and causes respiratory distress syndrome (RDS), which can be effectively treated with preparations which are lipid extracts of minced mammalian lung or lung lavage. Said preparations are known as modified natural surfactants and they are mainly composed of phospholipids (PLs) such as phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) and the hydrophobic surfactant proteins B and C (SP-B and SP-C).
For clarity, a list of PLs cited in this patent application follows:                phosphatidylcholine: PC,        phosphatidylethanolamine: PE,        phosphatidylglycerol: PG,        phosphatidylinositol: PI,        phosphatidylserine: PS,        sphingomyelin: SM,        1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol, generally known as dipalmitoyl-phosphatidylglycerol: DPPG,        1,2-dipalmitoyl-sn-glycero-3-phosphocholine, generally known as dipalmitoyl-phosphatidylcholine: DPPC,        1-palmitoyl-2-oleyl-sn-glycero-3-phosphoglycerol, generally known as palmitoyl-oleyl-phosphatidylglycerol: POPG,        1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine generally known as palmitoyl-oleyl-phosphatidylcholine: POPC,        1,2-dioleyl-sn-glycero-3-phosphoglycerol generally known as dioleyl-phosphatidylglycerol: DOPG,        1-palmitoyl-2-linoleyl-sn-glycero-3-phosphocholine, generally known as palmitoyl-linoleyl-phosphatidylcholine: PLPC,        1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphocholine, generally known as stearoyl-arachidonoyl-phosphocholine (SAPC),        1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (PAPC),        1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine, generally known as dipalmitoyl-phosphatidylethanolamine: DPPE,        1,2-distearoyl-sn-glycero-3-phosphoethanolamine, generally known as distearoyl-phosphatidylethanolamine: DSPE,        1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine, generally known as dipalmitoyl-phosphatidylserine: DPPS.        
The glycerol moieties of the phospholipids are mainly esterified with long chain fatty acids (C14-C20) which in turn can be saturated (e.g. myristic, palmitic and stearic acid), monounsaturated (e.g. oleic acid) or polyunsaturated (e.g. linoleic and arachidonic acid).
Phospholipids containing as the characterizing residues neutral or zwitter-ionic moieties such as glycerol (PG), inositol (PI) and serine (PS) are known as acidic phospholipids. Other examples of acidic phospholipids are DPPG, POPG and DPPS.
Surfactants are usually administered to premature infants in the form of aqueous suspensions by instillation into the lungs through the trachea. They can also be administered to adults affected by various pathologies involving a severe pulmonary insufficiency such as adult respiratory distress syndrome (ARDS).
In order to have optimal properties from a bio-physical and pharmacological/therapeutic standpoint, a surfactant preparation should: i) effectively lower the surface tension; ii) have a good spreading rate; iii) have a low viscosity in order to allow the preparation of a concentrated suspension in an aqueous medium with optimal delivery and distribution properties at alveolar level, upon administration in a small volume by intratracheal instillation.
The capability of the surfactants of lowering the surface tension as well as other parameters such as the spreading rate can be tested in vitro using several methods, for example the “captive bubble method” as described in Schurch, S., Bachofen, H. Goerke, J., Possmayer, H. (1989) “A captive bubble method reproduces the in situ behaviour of lung surfactant monolayers” J. Appl. Physiol., 67: 2389-2396.
One of the most important lipidic components of surfactant preparations is 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) as it forms a monomolecular film at the air-liquid interface during compression, probably undergoing phase transition (solidification) during surface compression, thereby stabilizing a system of alveoli with different sizes.
It is also generally recognized that acidic phospholipids are of paramount importance in order to obtain a good activity since they favour the spreading of DPPC.
On the other hand, high concentrations of a saturated PL such as DPPC can affect other properties of the lipid suspension, such as the viscosity of the preparation.
The possibility of preparing concentrated suspension in a small volume is a feature of particular importance for the administration of surfactants to low-weight newborns by endotracheal instillation.
The modified natural surfactants available for therapeutic use, have a concentration generally comprised between 25 and 50 mg/ml. Only one of them, “Curosurf®”, is available at a higher concentration, that is 80 mg/ml.
Surfactant preparations obtained from animal tissues present anyway some drawbacks, like their availability in limited amounts, the complexity of the production and sterilization processes and the relevant production costs: as a consequence, many efforts have been made to prepare synthetic surfactants.
According to Wilson (Expert Opin Pharmacother 2001, 2, 1479-1493), synthetic surfactants are distinguished in:
“artificial” surfactants devoid of surfactant proteins, simply consisting of mixtures of synthetic compounds, primarily phospholipids and other lipids that are formulated to mimic the lipid composition and behaviour of natural surfactant; and
“reconstituted” surfactants which are artificial surfactants to which have been added surfactant proteins isolated from animals or manufactured through recombinant technology such as those described in WO 95/32992, or synthetic surfactant protein analogues such as those described in WO 89/06657, WO 92/22315 and WO 00/47623.
The development of reconstituted surfactants has to a large extent been focused on the surfactant protein analogues while the lipid composition has obtained less attention.
Not much information about the composition and lipid mixture concentration of synthetic surfactants under development is available.
In a phase III, pivotal, masked, multinational, randomized trial, 5.8 ml/kg or 175 mg/kg of Surfaxin® were administered for comparing its properties vs Exosurf® in preventing RDS (Moya F. et al Abstract n. 2643 of the Annual Meeting of the Paediatric Academy Societies, San Francisco, May 1-4, 2004). From these values it can be extrapolated that Surfaxin® was administered at a concentration of 30 mg/ml.
Surfaxin®, also reported as KL4-Surfactant is a synthetic peptide-containing surfactant prepared by mixing the phospholipids dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) in a 3:1 ratio by weight with palmitic acid (PA), 15% by weight compared with the phospholipids, in an organic solvent.
In a previous study it was also utilized at two concentrations: 26.6 mg/ml and 35 mg/ml, expressed as phospholipid concentration (Am J Respir Crit Care Med 1996, vol 153, pp 404-410).